This invention relates to a method of injecting chemicals into a hydrocarbon reservoir for the purpose of increasing hydrocarbon recovery. More particularly, the invention pertains to the use of a combination sacrificial agent system of lignosulfonate and polyalkylene glycol to decrease adsorption of surfactants within hydrocarbon reservoirs.
One of the most vexing problems in the use of surfactant flooding for enhanced oil recovery is the frequent, substantial loss of surfactant due to adsorption on the formation matrix and precipitation by polyvalent cations such as calcium and magnesium. A significant percentage of surfactants are also physically entrapped within the pore spaces of the rock matrix. Of chief concern is surfactant adsorption on the formation matrix which significantly decreases surfactant flood efficiency, making it necessary to inject a greater quantity of surfactant and increasing the cost of any surfactant flood.
Additionally, most surfactants are satisfactory for surfactant flooding only if the calcium and magnesium concentrations of the formation water fall below about 500 ppm. Petroleum sulfonates, the most popular type of surfactants, precipitate where divalent ion concentrations exceed about 500 ppm. Such precipitation renders the sulfonates inoperative for recovering oil and in some instances, causes formation plugging.
The main cause of surfactant loss is adsorption within the formation due to physical contact of the surfactant with the formation matrix or entrapment within pores of the matrix. Surfactant systems contacting a sandstone matrix encounter a range of adsorptive sites. Although a number of adsorptive sites in sandstone are negatively charged, there are also a number of positively charged sites. For example, the surfaces of clay platelets, which are invariably present in sandstone reservoirs, have negatively charged sites, and the edges of clay platelets have positively charged sites. The presence of small amounts of limestone or dolomite in sandstone will additionally introduce positively charged adsorptive sites.
The most promising approach for reducing the amount of surfactants retained by the formation matrix has been to use sacrificial agent compounds, either in a preflush solution injected before the surfactant-containing solution, or in the surfactant solution. The compounds are sacrificial in that their adsorption on the formation matrix and entrapment within the pore spaces of the formation reduces the loss of the more expensive surfactants, solubilizers and polymers contained within the surfactant solutions.
Various chemicals have been employed as sacrificial agents to decrease the adsorption of surfactants or to tie up polyvalent cations and prevent them from precipitating surfactants from the flooding medium. Lignosulfonates form one class of compounds which have been found to have excellent properties as sacrificial agents. They are economically attractive because they are by-products of the pulp industry. Supply is plentiful and product costs are much less than the costs of surfactants employed in enhanced oil recovery floods.
The use of various lignosulfonates has been disclosed extensively in the literature. U.S. Pat. Nos. 4,157,115 and 4,271,906 disclose several methods of using aqueous solutions of lignosulfonate salts as sacrificial agents. The use of oxidized lignosulfonates is disclosed in U.S. Pat. Nos. 4,133,385 and 4,196,777. Chrome lignosulfonates as sacrificial agents are described in U.S. Pat. No. 4,142,582. U.S. Pat. No. 4,172,497 discloses the use of lignosulfonates carboxylated with chloroacetic acid and U.S. Pat. No. 4,172,498 discloses sulfomethylated lignosulfonates as sacrificial agents. U.S. Pat. No. 4,479,542 describes a sacrificial afterflush method employing lignosulfonates.
U.S. Pat. Nos. 4,452,308 and 4,589,489 teach the use of polyalkylene glycols as sacrificial agents. Both patents discuss low molecular weight polyalkylene glycols having molecular weights below 1200. U.S. Pat. No. 4,627,494 discloses the use of a mixture of at least two sacrificial agents, wherein one sacrificial agent is a lignosulfonate and the second agent is an alkoxylated starch or cellulose.
Changes in surfactant flooding in the last decade have shown that mobility control polymers are required to improve the sweep efficiency, and thus the oil recovery efficiency, of a surfactant flooding system. U.S. Pat. No. 4,271,907 among others discloses the addition of a mobility control polymer to a surfactant system to increase oil recovery efficiency. However, mobility control polymers are incompatible with many sacrificial agent systems. For example, a mixture of alkoxylated cellulose or starch with high molecular weight mobility control polymers such as polysaccharides or polyacrylamides results in phase separation. Phase separation renders surfactant flooding systems useless for efficient oil recovery.